Depression is associated with low levels of serotonin and norepinephrine. These are neurotransmitters, or chemical messengers, known to affect mood.

Serotonin is sometimes called a “feel-good” chemical because it’s associated with positive feelings of well-being. Norepinephrine is related to alertness and energy.

It’s believed that SNRIs help treat depression by keeping up the levels of these two chemical messengers in your brain. They do this by stopping serotonin and norepinephrine from going back into the cells that released them.

Another SNRI known as sibutramine (Meridia) was pulled from many countries, including the United States and Australia, in 2010. Marketed as a weight-loss drug, it was associated with multiple cases of cardiovascular events and stroke.

Levomilnacipran and milnacipran are only available as brand-name drugs. The others are available as both brand-name and generic drugs.

Milnacipran is used to treat fibromyalgia. It isn’t approved by the U.S. Food and Drug Administration (FDA) to treat depression, but your doctor may prescribe it off-label for that purpose.

OFF-LABEL DRUG USE Off-label drug use means that a drug that’s been approved by the FDA for one purpose is used for a different purpose that has not been approved. However, a doctor can still use the drug for that purpose. This is because the FDA regulates the testing and approval of drugs, but not how doctors use drugs to treat their patients. So, your doctor can prescribe a drug however they think is best for your care.